Saturday, August 29, 2009

High ISO is bad right?

One of the mantras you hear over and over is that to avoid noise, you should shoot as as low an ISO as possible because "high ISOs cause noise". However, this is based on a misunderstanding of how the physics of these sensors work. It turns out that actually, for equal exposure (i.e. shutter and aperture), you want to use as high an ISO as you can. The noise will be almost identical but you will be more evenly using the analog to digital converter. I know this sounds heretic, but it is very easy to prove. I took a set of shots all at the exact same exposure but with changing ISOs from 200 to 1600. I converted them in Lightroom 2.4 using default settings, but with the blacks set to zero and the noise reduction and sharpening turned off. I had to add exposure compensation to the low ISO shots in order to bring them up to the same level as the ISO 1600 one. Mouseover to see the other image (first time this might take a few seconds as the alternative image will have to be loaded).

D300, 50 mm/f 1.8, 1/5 s at f 3.2.

So can you guess which one is which? The slight color difference is coming from the hue shift inherent in the Adobe profiles (Adobe standard in this case) and are not real. You can get rid of those by untwisting them. But the noise is basically identical and the detail in the image is identical. The answer is that the mouseover image is ISO 200.

Of course, if you correctly expose the ISO 200 image, you will get far lower noise. Right now, it was 4 stops underexposed. This has nothing to do with the ISO, but everything to do with the physics. Correctly exposing at ISO 200 means exposing 4 stops longer. This means 24 more light hits the sensor. Since incoherent photons follow Poisson statistics, this means that the noise increases by a factor 22 (=sqrt(24)), so the signal to noise ratio increases by a factor of 24/22=4. What matters in modern digital cameras that have little read noise and thermal noise (as long as you're not doing very long exposures) is simply the amount of light hitting the sensor. To get lowest noise, you should expose longer or increase the aperture. At the same time, to avoid blowing out the highlights, this means lowering ISO. Some folks make the conclusion from this that this means that lower ISO means lower noise or the inverse that high ISO means higher noise. However, as I showed above, this is simply not true. For a single camera, it is simply the increased exposure that you need at lower ISO, not something inherent to low ISO itself. So if you have a fixed aperture and fixed shutter, it is probably better to increase the ISO until you get correct exposure, which in digital often means avoiding blown highlights that you care about, than it is to underexpose at lower ISO.

ETTR?

Conversely, if you only care about noise and have ample time, you should choose your lowest native ISO and increase your exposure until you start blowing out highlights that you care about. This is the ETTR mantra that you often hear (but that people often get the physics off wrong). See here for the canonical article that is right about the outcome, but wrong about the reason for the observation. It has nothing to do with digital binning steps, but everything with photon shot noise as I explain above. Note however, that if you are in a large dynamic range situation such as a setting sun on a landscape, you are far better off bracketing a few exposures and combining them afterwards or using a graduated ND filter than using the ETTR technique. In such a setting, ETTR usually means increased noise. This is because you have to expose to not ruin the very important highlights (e.g. the mountain illuminated by the setting sun), which means underexposing the darker foreground considerably. This induces lots of shadow noise. Using a graduated ND, or exposing separately for the highlights and the foreground solves this problem.

EDIT: I learned a lot by reading an article about this a while ago. So I spend a few minutes trying to tease it out of Google again. Here is a link to the pertinent part. The author gives the correct explanation behind ETTR and reaches the same conclusion as I reached above. To quote this excellent article:

Bottom line: Read noise at high ISO is much smaller than read noise at low ISO, in terms of the error in photon counting that it represents. Thus, better image quality is obtained for using the highest ISO for which the signal is not clipped.

9 comments:

Jao, I read through the chapter that you linked. After reading it, it seems like the takeaway is that when you need detail in the shadows, bump up your ISO, but when you need the highlights for detail, go with the lower ISO levels for the headroom (unless you are doing long exposures when the lower ISO improves the image).

No, to use the untwisted profiles you have to shoot RAW. You cannot influence the RAW engine in the camera at this level unfortunately. You do not need to convert to dng. The profiles work just fine with NEF.

Hmmm, well I don't want to have to shoot in NEF either, so it looks like I'm stuck with hue shifting for the time being... if only there was an option in Lightroom to lock the hues for the exposure slider... perhaps I should write in and suggest it...

Marcus, the exposure slider in Lightroom should not affect hue when looking at a jpeg, but it could certainly affect saturation. I rarely shoot jpeg, so I have little experience with it though. The few jpegs I have seem invariant under the exposure slider.

I don't really notice that (just spend some time sliding a portait jpeg back and forth) but I do notice that the jpegs break up really quickly and blow out with an ugly atomic look on skintones, where similar RAW files work much nicer. At moderate exposure adjustments, the result in LR looks very similar to doing a levels in Photoshop, so I doubt Lightroom is doing funny stuff. I think that if you shoot RAW in these cases you will probably get a far better control over the result. It sounds like you're perhaps running into a limitation with jpegs and you might have to get the exposure absolutely perfect in camera if you choose to shoot jpeg. Maybe a good thing to test is to shoot RAW+jpeg for a little and see what works best.

I had no idea the issue was so complex. I used to expose more to the right and use the lowest ISO possible. Of course some situations are not ideal for low noise or even appropriate shutter and aperture settings.